In converters, the output voltage should remain constant even in the presence of fluctuations of the input voltage, the load, and the parameters, such as temperature and spread of the values of components. This has been achieved by using a closed loop regulation technique. Current mode control is very commonly used, and a typical implementation block diagram is shown in FIG. 1. The output voltage of the converter, or more often a scaled fraction thereof, is compared with a reference value VREF, and the difference is amplified by an error amplifier E/A.
The amplified signal is compared with information coming from the current sensing resistor RSENSE and is processed by an appropriate driver. With this technique, the drain current is controlled during every PWM cycle and its amplitude is modulated by the control loop in order to keep the output voltage constant. Numerous commercially available controllers use the circuit of FIG. 1, which allows for integration of the blocks E/A, PWM and the control logic of the power switch of the converter. Notably, an external compensation network realized with a resistor and a capacitor or with a combination thereof may be externally connected to the output of the error amplifier. Moreover, for realizing the error amplifier E/A, a relatively large silicon area is used, besides using bias currents and associated circuits.
There are other ways of exploiting the advantages of current mode control without using an error amplifier. This is the case of the controller implemented in the family of commercial devices VIPerX2, as available from the present application's assignee, STMicroelectronics of Geneva, Switzerland, which is depicted in FIG. 2. The clamp current is proportional to the current input through the control pin FB according to a typical control law. This approach may imply the need of a voltage reference (TL431, a shunt regulator referred to 2.5V or a Zener diode) in order to fix the output voltage. Moreover, the FB pin may be properly controlled in current mode through a photo-coupler in insulated applications, and in non-insulated topologies, the regulation may be done by connecting a Zener diode in the feedback path to the pin FB.
The use of a voltage reference and of a photo-coupler implies a significant cost, and the alternative embodiment of using a bipolar Zener diode may make the system less precise because of a significantly larger tolerance of the reference and of “spread” of parameters in function of temperature (that may only partially be nullified by realizing specific compensation circuits). Another approach may include using a simple ON-OFF control, as implemented in the devices of the family LinkSwitch-TN of Power Integration, which is schematically illustrated in FIG. 3.
In this case, the FB pin is the output of a low impedance circuit at 1.65V connected as a source follower. At the beginning of each switching cycle, if the current input to the pin is smaller than a certain value, for example, 49 μA, the control logic may enable the driver, and the consequent switching cycle may take place. Vice versa, if the current is greater than the threshold, the control logic disables the driver, and the switching cycle may be omitted. This technique may make the controller extremely simple to use and intrinsically stable, besides ensuring a very good response to transients. However, because the switching cycle starts always at the maximum clamp current, even at very small loads only when the average switching frequency varies, the system is not suitable for flyback converters, because acoustic noise may be generated on the transformer at low output load conditions.